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Resistive & Permanent Magnets for Accelerators. J. G. Weisend II National Superconducting Cyclotron Lab Michigan State University. Introduction. Magnets are an important part of accelerator technology Many accelerators are “mostly magnets” - PowerPoint PPT Presentation
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Resistive & Permanent Magnets for Accelerators
J. G. Weisend IINational Superconducting Cyclotron Lab
Michigan State University
Introduction• Magnets are an important part of accelerator
technology– Many accelerators are “mostly magnets”
• Particle accelerators operate with charged particle beams (e.g. electron, protons, ions, positrons, antiprotons etc)– Interaction of these charged particles and the
magnet field allows manipulation of the beam
2J. G. Weisend II
BveF
Uses of Magnets in Accelerators• Bending or Directing of Beam
– Dipoles
• Focusing of Beam– Quadrupoles
• Beam Correction– Sextupoles and higher
• Beam “Compression” or Energy Separation– Chicanes using dipoles
• Background field for detectors– Solenoids
• Moving charged particles to create synchrotron light– Wigglers, Undulators
A key issue is field uniformityHaving the right field in the right place
Magnetic Design• Modern magnets are designed using a variety of
modeling software (both 2D & 3D)– ANSYS– MAXWELL– TOSCA– POISSSON and PANDRIA– OPERA
• Programs are based on standard EM theory and most use Finite Element Analysis (FEA)
• Programs take into account material properties• Some approximate equations will be given later
Magnet Technology Choices• Permanent• Resistive• Superconducting
Relative MeritsType Advantages Disadvantages
Permanent CompactLow cost ( in small low field magnets)No utilities requiredNo maintenanceSimple to operate – no adjustments requiredCan result in very precise fields (FEL undulators)
Constant field (mostly)Limited in field
Resistive Variable fieldNo need for complicated cryogenic or vacuum systemsCan be built in house or through existing industrial baseRelatively low capital cost
Limited in field (up to ~ 2 T)May require large amounts of electrical power and cooling water – complicatedPossible tritium issuePossible large operating costs for power & water
Relative MeritsType Advantages Disadvantages
Superconducting High fieldVariable fieldLower operating costsReliabilityCold beam tubes yield very high vacuumsCan be made compact
High capital costsLimited industrial baseRequires complicated ancillary systems – cryogenics, vacuum, quench protection
•Technology choice depends on accelerator requirements•All design is compromise
•Requirements for higher energy beams frequently dominate•Permanent and resistive magnets discussed in this talk•Next talk discusses superconducting magnets
Permanent Magnets • Much of this material is taken from Jim Volk
Fermilab Physicist• Permanent magnets are used in very
sophisticated ways in particle accelerators– Fermilab 8 GeV recycler ring is almost entirely
permanent magnets– Precision fields in undulators
• Used where high field strength or field variation is not a requirement– Required field must be well known and unchanging
Permanent Magnet Materials
Note 10,000 Gauss = 1 T CourtesyJ. Volk
Hallbach Array
CourtesyJ. VolkThe easy axis is the direction of field in material
Hallbach Dipole
CourtesyJ. Volk
Hallbach Quadrupole
CourtesyJ. Volk
2 proposals for adjustable field permanent magnets
Hallbach Ring QuadRotating out ring of magnets Changes fieldProposed for NLC
Moveable magneticMaterial adjusts field of quadS. Gottschalk et al.
CourtesyJ. Volk
Effect of Temperature• Field Dependence: Depending on the material, effect
can be significant with moderate temperature change (tens of degree C)
• Even smaller temperature changes can impact alignment & thus function of FEL undulators
• Solutions• Tight environmental controls (LCLS, FLASH)• Temperature compensated design to reduce field
dependence
• Actively cooled devices (s/c & resistive magnets) may have fewer issues - cryostat
ExampleFermilab Recycler Ring
CourtesyJ. Volk
Recycler Design
CourtesyJ. Volk
Recycler Gradient Magnets
CourtesyJ. Volk
Recycler Quads
CourtesyJ. Volk
Note the recycler was ultimately successful
PM Undulators• Moves particle beam back and forth to create synchrotron
light & possibly FEL• Frequently use PM (though resistive & s/c versions also
exist)• Many different types exist ( transverse, helical, with or
without iron)
F. Robles LBNL
PM Undulators• The magnets should be as close to the beam as possible• The precision of the magnetic structure is very important to
success– Requires high mechanical and thermal stability
• Deformation limits of a few micrometers are common• Tends to result in large support structures and tight environmental controls
– Requires precise magnetic measurements & precision alignment
• A wide variety of these devices have been built or proposed
Some Examples
Undulator Design from HASYLabDESY
Undulators for FLASH FELDESY
Cryogenic Permanent Magnet Undulator
• Lowering temperature to 140 K increases field by ~ 13 - 25 % to about 0.5 T(rare earth magnets NdFeB and PrFeB) Radiation damage a concern in NdFeB
• May also help in thermal stability
From “DEVELOPMENT OF A PRFEB CRYOGENIC UNDULATOR AT SOLEIL”
C. Benabderrahmane et al. Proceedings of IPAC’10, Kyoto, Japan
Resistive Magnets• Electromagnets
– Excitation is provided by current carrying conductors ( generally Cu)
– Field is shaped by Iron poles– Field in iron is less than the iron saturation value– Resistive losses in the conductor frequently
require water cooling
• Basis of electromagnets is the Biot-Savart law
R
IB
20
Examples of Ideal Magnets
Dipole
Quad Sextupole
From“Iron Dominated Electromagnets”J. TanabeSLAC-R-754
Some Approximate Equations(assumes iron not saturated)
Dipole Quadrupole Sextupole
Field (B)
Power Dissipated
h
NIB 02
2
02
r
NI
dr
dB
30
2
2 6
r
NI
rd
Bd
0 avgBhjl
P 0
2 )/(2
drdBjlr
P avg0
223 )/(
rdBdjlr
P avg
Where I = current, N = number of turnsh = gap height, r = radius from centerlavg = average length of turnresistivityj = current density = efficiency ( ~ 0.99) = 4 x 10-7
From : Th. ZicklerBasic Magnet DesignCERN Accelerator SchoolBurges – 2009
http://cas.web.cern.ch/cas/Belgium-2009/Bruges-after.html
Comments on Resistive Magnet Cooling
• If the power dissipated via Ohms law is too high than the magnet will over heat and melt
• Thus – the current density must be keep low – yielding
very large conductors or– Cool conductors actively with flowing water
• Flow rates and thus pumping costs can be very high• LCW is generally used (expensive)• Tritium may be produced and have to be dealt with
Fabrication of Resistive Magnets• Proper fabrication of resistive magnets is a very
complicated issue and success requires a great deal of experience
• A very detailed and useful guide here is “Iron Dominated Electromagnets” J. Tanabe, SLAC-R-754 (included in your materials)
• Basically, fabrication can be divided into 3 areas : the yoke, the coil and assembly and installation
Yoke Fabrication• A fundamental requirement is that resistive
magnets should have reproducible performance under excitation & have limited multipole fields
• This requirement is met largely by the iron (steel) yoke – Avoid saturation in the yoke
Yoke Laminations• Yokes are generally made out of laminations to reduce eddy currents thus improving field quality and reducing hysteresis• Laminations are generally shuffled or sorted to allow similar material properties in each magnet• Laminations are carefully shaped to avoid saturation and to deliver the correct field shape•Laminations can be assembled using glue, welding or via mechanical fasteners•For small numbers of magnets with low AC use solid material rather than laminations is frequently used
Yoke AssemblyFrom“Iron Dominated Electromagnets”J. TanabeSLAC-R-754
Coil Fabrication• Wind copper to produce desired field• Insulate via glass tape mylar or other materials
to prevent turn to turn shorts• Install water & electrical connections• Finished coils are typically potted in epoxy to
prevent damage, maintain the coil shape and prevent shorts to ground
Assembly & Installation• The coil, yoke, beam tube and other
assembled together• The magnet fields are carefully measured and
references are installed to allow precise alignment of the magnet with the beam center
• Other tests such as checking water flows and HiPotting ( testing for shorts to ground) are done
Example 1Fermilab Main injector Dipole
• Principle bending magnet in Fermilab Main Injector
• Required to ramp from 0.1 T to 1.73 T at 2.4 T/s
• 344 dipoles were required
Example 1Fermilab Main injector Dipole
From “Design Considerations & Prototype Performance of the Fermilab Main Injector Dipole” D. Harding et al. (PAC 1991)
Example 1Fermilab Main injector Dipole
From “The Design & Manufacture of the Fermilab Main Injector Dipole” B. Brown. (EPAC 1992)
Main Injector and Recycler Magnets in Fermilab Tunnel